Understanding the Ken Burns Effect

The Ken Burns effect is a visual transition used in filmmaking and video editing to make an image or video appear as if it’s being zoomed into or out of frame. This effect can be achieved using various techniques, including animation and transformation of the image layer.

In this article, we’ll explore how to create a Ken Burns effect on an UIImageView using UIKit and Core Animation.

Background

To understand the Ken Burns effect, let’s first look at how it works in general terms. When an image is zoomed into or out of frame, the camera appears to move closer to or farther away from the subject, creating a sense of depth and dimensionality. This movement can be achieved using animation, which involves manipulating the image layer’s transform properties over time.

In our case, we’ll use Core Animation to create a Ken Burns effect on an UIImageView. We’ll achieve this by animating the image layer’s transform properties, such as scale, position, and rotation.

The Problem

The question at hand is why the first animation (zoom-in) works perfectly, but adding a didStopSelector to the animation causes the method to be called directly, rather than after the animation has finished.

To understand this issue, let’s break down how animations work in UIKit. When an animation is started using UIView.beginAnimations, it begins executing immediately, and the animation delegate (in our case, self) receives callbacks at various points during the animation’s execution, such as when the animation starts (animationDidStart), when a frame is rendered (animationUpdate), and when the animation finishes (animationDidStop).

The problem arises because when we add a didStopSelector, it specifies a method to be called when the animation finishes. However, if we specify this selector in the initial UIView.beginAnimations call, it will cause the method to be called immediately, rather than waiting for the animation to finish.

Solution: CAAnimationGroup

The solution lies in using a CAAnimationGroup, which is a group of individual animations that can be combined into a single animation. When an animation group finishes, all its constituent animations also finish.

Here’s how we can modify our code to use an animation group:

- (void)beginKenBurnsEffect {

    // Create an array to hold our animations
    NSMutableArray *animations = [NSMutableArray array];

    // Add the zoom-in animation
    CAAnimation *zoomInAnim = [CABasicAnimation animationWithKeyPath:@"transform"];
    zoomInAnim.fromValue = [NSValue valueWithCGAffineTransform:CGAffineTransformScale(self.view_image.transform, 1.06, 1.06)];
    [animations addObject:zoomInAnim];

    // Add the move-in animation
    CAAnimation *moveInAnim = [CABasicAnimation animationWithKeyPath:@"position"];
    moveInInm.offset = 5.0;
    [animations addObject:moveInAnim];

    // Create an animation group and add our animations to it
    CAAnimationGroup *group = [CAAnimationGroup animation];
    group.animations = animations;
    group.duration = 5;

    // Add the animation group to the layer's animation cache
    [self.view_image.layer addAnimation:group forKey:nil];

    // Save the initial transform and center values
    self.origTransform = self.view_image.transform;
    self.origPoint = self.view_image.center;
}

- (void)endKenBurnsEffect {

    // Create an array to hold our animations
    NSMutableArray *animations = [NSMutableArray array];

    // Add the zoom-out animation
    CAAnimation *zoomOutAnim = [CABasicAnimation animationWithKeyPath:@"transform"];
    zoomOutAnim.toValue = self.origTransform;
    [animations addObject:zoomOutAnim];

    // Add the move-out animation
    CAAnimation *moveOutAnim = [CABasicAnimation animationWithKeyPath:@"position"];
    moveOutAnim.offset = -5.0;
    [animations addObject:moveOutAnim];

    // Create an animation group and add our animations to it
    CAAnimationGroup *group = [CAAnimationGroup animation];
    group.animations = animations;
    group.duration = 5;

    // Add the animation group to the layer's animation cache
    [self.view_image.layer addAnimation:group forKey:nil];

    // Save the final transform and center values
    self.origTransform = self.view_image.transform;
    self.origPoint = self.view_image.center;
}

Conclusion

In this article, we explored how to create a Ken Burns effect on an UIImageView using UIKit and Core Animation. We discussed the problems with adding a didStopSelector to an animation and provided a solution using a CAAnimationGroup. By combining individual animations into a single group, we can ensure that all animations finish simultaneously, providing a smooth and seamless experience for the user.

Additional Considerations

There are several additional considerations when implementing the Ken Burns effect:

• Timing: The timing of the animation is critical. A longer duration will result in a more pronounced effect, while a shorter duration will result in a less noticeable effect.
• Scaling: Scaling an image can be achieved using the CGAffineTransformScale function or by using Core Animation’s built-in scaling animations.
• Positioning: Positioning an image within its parent view can be achieved using Core Animation’s position animations or by manipulating the center property of the image layer.

By understanding how to implement these additional features, you can create a more sophisticated and engaging Ken Burns effect that enhances your app’s user experience.

Last modified on 2023-08-28